In electric generators provided with bar stator windings the bars are laid in radial slots of the stator core while their ends extend beyond said slots. In the end stator windings at the stator end faces, bar ends are connected, in accordance with the circuit diagram adopted for the particular generator, one to another by respective jumpers. The end connections of the bars are divided along the stator circumference into several alternating groups belonging to different winding phases, and every such a group comprises several end connections belonging to one winding phase. While a comparatively low potential difference exists between two adjacent bar end connections in one winding phase group during the operation of the generator, this potential difference being determined by the interturn voltage of one winding phase, the full line voltage of the stator winding can be present between adjacent extreme end connections of two neighbouring phase groups. In modern powerfull generators, the line voltage of the stator winding can reach a value of the order of 30 kilovolts. In order to avoid an electric breakdown between extreme end connections, they are, therefore, to be provided with means ensuring a reliable insulation of one such end connection from another. Furthermore, the arrangement used to insulate the end connections should at least not deteriorate the mechanical strength of the winding and, in particular, the end windings that can undergo rather high electrodynamical loads during the operation of the generator. At the same time the electrically insulating arrangement should reliably perform its functions during an adequately long time under various operating conditions of the generator, including its high-voltage testing, high-load operation and short circuits.
Known in the prior art are two basic types of electrically insulating arrangements for end connections of the stator winding bars, namely, the continuous-type arrangement that is a layer incorporating many turns of electrically insulating tape impregnated with respective substances, and thermally treated, and the barrier-type arrangement comprising insulating members of various geometrical shape installed between the end connections to be insulated and being certain types of barriers preventing the breakdown current flow. The use of the continuous-type arrangement complicates the production process of the electric machine and the repair thereof as well as does not allow to design an end winding, construction with the desired mechanical stiffness. The barrier-type arrangement are, therefore, used in powerful generators.
There is known in the prior art an electric-machine stator bar winding (cf., for example, the book V. V. Titov et al. "Turbo-generators. Calculation and construction", Leningrad, Publishing House "Energija," 1967, p. 209-210) wherein the end portions of bars projecting from the radial slots of the core have involute and straight-line sections, the latter having end connections. Said winding of the stator has, in the end windings, an arrangement for insulating the end connections of the extreme bars belonging to different phase groups of winding bars or, in short, the extreme end connections that are separated from each other by interphase regions. This arrangement incorporates barriers made as integral box-type screens from an insulating moulded material. The box-type screens are so installed that they encompass the extreme end bar connections and extend along a portion of straightline sections of bar ends, that are connected, the interior of the box-type screens being filled with an insulating compound. In this stator winding, the insulation of extreme end connections is provided primarily by the dielectric strength of the box-type screen walls and the air gap in the interphase regions, wherein the electric field is comparatively little non-uniform.
However, in such a construction, the insulation reliability becomes insufficient in electric machines with a high unit power and an increased rated voltage since, firstly, the material of the box-type members has a comparatively low dielectric strength and is apt to a relatively quick thermal and electrical ageing and, secondly, the box-type screens cover only partially the straight-line sections of the end portions of the bars, that are insulated and very little envelop the basic insulation of the bars. An electrical breakdown of the air gap can, therefore, easily originate in the event of dirtening or moistening of the interphase region for any reasons with the development of flashover of this region when the breakdown current flows over surfaces of the electrically insulating members and the basic insulation of the bars. The probability of the flashover increases still further when dielectric spacer members causing a re-distribution of the voltages and a sharp increase in the non-uniformity of the electric field in the interphase regions are introduced between the box-type screens in the interphase regions in order to provide the mechanical stiffness of the end windings. Due to this reason this construction cannot provide an increased stiffness of the end windings without a certain decrease in the reliability of the electrical insulation of the extreme end connections of the winding bars. The inadequate stiffness of the end windings leads to their vibrations generated under the action of variable electrodynamical loads during the operation of the electric machine. The vibrations of the end windings can lead in turn, especially in powerfull electric machines, to breaks in soldered joints of the bar end connections, to an abrasion and crumpling of the insulation at the places of securing of the bar end portions and of their emergence from the core slots and to a fatigue damage to elementary copper conductors of the winding bars. This can cause in turn an emergency and failure of the electric machine.
Also known in the prior art is another electric-machine stator bar winding whose end bar portions have involute and straight-line sections with end connections as well (cf., for example, the article by K. Wanke "Turbogeneratoren fur Kernkraftwerke" in the magazine "AEG und Telefunken technische Mitteilungen", Berlin, 1969, No. 1, p. 1-10). This stator winding also comprises, in the end windings, an arrangement for the electric insulation of the end connections of the extreme bars included into different phase groups of winding bars. The insulating arrangement comprises integral dielectric box-type screens having a rectangular cross-section similar through the screen length, and each of the screens encompasses an extreme end bar connection. The box-type screens are installed in pairs in such a manner that they have adjacent walls in each interphase region of the end windings. The box-type screens have such a length that each of them covers completely the connected straight-line sections of bar end portions. An additional dielectric flat screen is installed between each pair of the box-type screens in the central port of the interphase regions. In this winding, the electric insulation of extreme end connections is provided not only by the dielectric strength of walls of the box-type screens and the air gap in the interphase regions, but also by that of the additional flat screen.
However, the use of the insulating arrangement described hereinabove and possessing acceptable insulating properties for extreme end connectons in the known prior art stator winding led to a complication of the construction of the end windings. This complication is expressed in the facts that, firstly, the length of the straight-line sections of the bar end portions is increased in order to reduce the probability of flashover by providing a larger coverage of the basic insulation of these sections by the box-type screens, leading to an increase in the total length of the electric machine and, secondly, the end portions of the bars in each phase group have additional bents towards the electric-machine housing in order to provide larger width of the interphase regions. Furthermore, in this insulating arrangement it is difficult to provide a reliable securing of the additional flat screen, hindering the provision of stiffness of the end windings and, hence, their resistance to vibrations. It is also worth to mention the inconvenient and complicated installation and removal of the box-type screens during the mounting and dismantling of the end windings, when the electric machine is built or repaired. This all complicates the manufacture of the elecric machine with such a bar stator winding and makes it more expensive.